Welcome to TTF!.
Before doing anything, make an Explorer, not Home, backup of your unit's contents.
Then connect to Home for updates and go to Tools-->Latest Map Guarantee. you should be offered version 850 of your map (doubt it'll come with that version installed).
Download, install (it will take a long time...) and make another Explorer backup to a different folder.
Then use Home to Update My unit to get mapshare changes and perhaps a quickgpsquickfix file.